1. Technical Field
The present invention relates in general to drill bit junk slots and, in particular, to an improved system, method, and apparatus for alleviating balling with variable junk slot depths in PDC drill bits.
2. Description of the Related Art
Polycrystalline diamond compact (PDC) drill bits have a common bit body and a plurality of fixed blades. Each pair of adjacent blades is separated by an elongated gap that is commonly referred to as a junk slot. Traditionally, PDC bits use the same junk slot depth for all junk slots. This design practice was acceptable when drill bits were created on a drafting table or with a two-dimensional computer aided (CAD) design system. However, with current three-dimensional CAD and computer aided manufacturing (CAM) systems, and numerically controlled machine tools the design of the bit body can be very flexible.
Drill bit “balling” occurs when the cuttings generated by a drill bit clog the junk slots such that removal of additional formation is impeded. Balling adversely affects the overall performance of drill bits and is generally recognized to be responsible for a significant reduction in the rate of penetration (ROP) of drill bits. Three factors that contribute to balling include: hydraulics (e.g., flow patterns, mud properties, etc.), operations (e.g., weight on bit, bit rotational speed, formation, etc.), and mechanical factors (e.g., a bit's physical design, cutter selection, etc.). Eliminating or controlling these factors directly affects the balling characteristics of any specific PDC bit.
Historically, the analysis of PDC drill bits with regard to balling has been focused on the operational parameters drilling mud rheology, flow rate and HSI. Although these two parameters have been shown to affect maximum penetration rates, other controllable variables have been identified that also contribute to bit balling. In addition, current models developed to predict ROP based on flow rate and/or HSI are not accurate, and no known model can establish the theoretical ROP limit for a given PDC drill bit. Thus, an improved drill bit design for reducing or eliminating balling would be desirable.